The present invention relates to a spectrometer or filter device. In particular, the present invention relates to a spectrometer device in which the wavelengths of a beam of photons are controllably detected and measured in relative intensity after being distributed to a set of photon receptor elements via a thin metal foil. Further, the present invention relates to spectral information in the source beam of photons that is subject to differentiation by wavelength and distance within the exponential field within one wavelength of the foil. The separation of wavelengths into multiple parts occurs by the stimulation of surface plasmons in the metal foil at the Brewster angle for each incident wavelength or by detection of the decay length of an evanescent field. An embodiment of the invention employs surface plasmons and provides filtering of all wavelengths not incident at the Brewster angle due to the high reflectivity of the metal foil at angles other than Brewster""s angle.
The use of surface plasmons to filter and hence to separate wavelengths of photons by using a supplementary dielectric second layer has been described in U.S. Pat. No. 5,986,808 by Wang and in U.S. Pat. No. 6,122,091 by Russell, both herein incorporated by reference. Each of these requires a second layer of dielectric beyond a metal foil that is essential to the phenomena considered and the filtering is not therefore solely dependent upon the surface plasmons. Nor is the case considered in these references one in which the surface plasmons are detected within one wavelength of the metal foil.
Similarly the use of surface plasmons in controlling photons by using a second dielectric layer has been described in U.S. Pat. No. 6,034,809 by Anemogiannis and is herein incorporated by reference. Anemogiannis teaches optical plasmon-wave attenuation and modulation structures for controlling the amount of coupling between a guided optical signal and a surface plasmon wave, but the second-layer required must be controlled by another device so as to cause a change in optical index of the second layer. Anemogiannis employs no condition in which the surface plasmons are associated with spectrometry or filtering without the presence of the controlling dielectric layer.
In summary, prior art has the necessity of a special second layer and the lack of use of surface plasmons in the exponentially decaying electric field provided by the surface plasmons within one wavelength of the surface. Therefore, these comprise prior art utilizing different physical phenomena to actually act upon the photons than is described in the present invention. Further they do not provide a means of differentiating the signal so as to determine distance to the surface.
The following background publications are herein incorporated by reference:
1. E. Kretschmann, E., Rather, H., Z. Naturforsch, 216, 398-410, (1968).
2. Otto, A., Z. Physik, 216, 398-410, (1968).
3. Welford, K. R., et al., xe2x80x9cCoupled Surface plasmons in a Symmetric System,xe2x80x9d Journal of Modern Optics, Vol. 35, No. 9, Pp.1467-1483, 1988.
4. Hoyt, Clifford C., xe2x80x9cTowards Higher Res, Lower Cost Quality Color and Multispectral Imaging,xe2x80x9d Advanced Imaging, pp. 53-55, April 1995.
5. Kajenski; xe2x80x9cTunable Optical Fiber Using Long-Range Surface Plasmonsxe2x80x9d; Society of Photo-Optical Instrumentation Engineers; Vol. 36, No. 19 Pp. 1537-1541, May 1997.
6. Wang, Yu, xe2x80x9cVoltage-induced Color Selective Absorption with Surface Plasmons,xe2x80x9d Appl. Phys. Lett., Vol. 67, No. 19, Pp. 2759-2761, Nov. 6, 1995.
7. Caldwell et al.; xe2x80x9cSurface-Plasmon Spatial Light Modulators Based on Liquid Crystalxe2x80x9d; Applied Optics; Vol. 31, No. 20; Pp. 3880-3891, Jul. 10, 1992;
8. Jung et al.; xe2x80x9cIntegrated Optics Waveguide Modulator Based on Surface Plasmon Resonancexe2x80x9d; Journal of Lightwave Technology; Vol. 12, No. 10, October 1994; Pp. 1802-1806.
9. Lozovik Y E; Merkulova S P; Nazarov M M; Shkurinov A P., xe2x80x9cFrom two-beam surface plasmon interaction to femtosecond surface optics and spectroscopyxe2x80x9d Physics Letters A, Vol 276, Iss 1-4, Pp. 127-132, Oct. 30, 2000.
10. Challener W. A.; Edwards J. D., McGowan R. W., Skorjanec J., Yang Z., xe2x80x9cA multilayer grating-based evanescent wave sensing techniquexe2x80x9d, Sensors and Actuators B-Chemica, Vol 71, Iss1-2, Pp. 42-46, Nov. 15, 2000.
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The tunneling of photons from a region of total-internal reflection is engendered by a probe or detector situated within one photon wavelength of the reflecting surface. The tunneling signal received is converted immediately thereafter to an electronic signal after being transmitted by an optical fiber or waveguide to a detector. This principle has long been used in phenomena of frustrated total internal reflection in the optical region of the spectrum in the case without a metal foil. The probe or detector is positioned with modern piezoelectric crystals to an accuracy of 0.0005 nm by using data on the ratio of the derivative of the signal with respect to wavelength to the derivative with respect to distance. If an unknown wavelength is introduced to the system, the ratio of the aforementioned derivatives can be thence remeasured to produce the value of the wavelength. If multiple wavelengths are introduced in combination, then a second photonic element is required in order to delineate the wavelengths and permit the above-mentioned measurements to obtain the intensity of each wavelength relative to the calibration wavelength. This additional element is a thin metal foil (typically 10-100 nm thick) placed on the reflecting surface. Each wavelength will induce surface plasmon quanta in the foil at an angle unique to that wavelength (Brewster""s angle) with a tolerance of milliradians. The surface plasmon field is of the same functionality (exponential in the tunneling gap) as the evanescent field in the original case, but tunneling now occurs only for a single wavelength with a degree of broadening due to damping of surface plasmons of the combined waves. The angle of incidence is thereafter successively changed by minute amounts in order to permit successive wavelengths and provide a tunneling signal. In this way a complete spectrum of the incident photons is obtained by repeated measurement of the derivative ratio or by simultaneously doing so with multiple probes or an array of detectors. A probe is produced by etching silicon dioxide on a silicon wafer or in the core of an optical fiber. Simultaneous measurement at many wavelengths can alternatively be used if the incident photons are dispersed in wavelength to any given degree if an array of charged-coupled devices or other solid-state devices are formed into a compact two-dimensional array and placed in the near zone of the foil. The resulting probe or detector is driven to resonance using similar electronics to that used for other micromechanical systems (e.g., micro-cantilevers, membranes, etc.). By tracking the bending and resonance behavior of the probe using the tunneling signal, the probe simultaneously functions as a sensor in a large variety of sensing applications. Therefore, the spectrum of a targeted sample is simultaneously obtained while sensing other properties of the sample. The sample may be an element or compound in fluid form, a biological material, or it may simply be desired to measure a physical property of the environment during the process of acquisition of a spectrum. Spectroscopies enabled on silicon by this device include all of the ultraviolet, visible, and infrared spectroscopies, Raman spectroscopy, and photometry. A special advantage is the high stray-light rejection factor provided by the metal foil for wavelengths not incident at the Brewster angle.
One embodiment of the invention is a microoptoelectromechanical integrated spectrometer with a photonic element assembly having metal foil removably disposed on a first transparent substrate surface, the substrate having no foil on any other surface. A means is provided for directing source photons that are reflected from or transmitted through a sample, over a range of angles of incidence, into the transparent substrate and onto the metal foil such that source photons are incident at the Brewsters angle. A means is also provided for detecting an induced exponential field in the metal foil. A means is also provided for relating the induced exponential field to a known exponential field for the sample and determining the identity of the sample. The spectrometer performs ultraviolet-to-visible-to-infrared spectroscopy using photon tunneling and surface plasmon excitation.